Not sure your questions have been answered, so I’ll jump in - I’ve been sitting for just over two years now, so still quite new in the scheme of things.
Diversity will very much vary by area; the ideal is that the bench should reflect the area it serves, but I’m sure most skew more towards older and white - and probably male.
My area has very low levels of visible diversity, but the bench has a slightly broader mix than the area. The Magistrates’ Association has groups for young magistrates (18+), people of colour, people with disabilities and LGB+ people. The groups all operate individually but also work together. All magistrates are expected not just to operate without bias (we have unconscious bias training) but to challenge discriminatory language or behaviour if we encounter it.
Length of deliberation varies, but generally with a guilty plea will take no more than ten minutes. So, for example, with a guilty plea for drunk driving, unless the levels were extremely high, you’re generally considering the level of fine and length of disqualification. The Sentencing Guidelines will give a starting point and range, so it’s a matter of deciding (and majority agreeing) where the offence sits within the range. Also, you’ll have been thinking about where you feel the offence sits while listening to the prosecution and defence/defendant, which speeds up deliberations. With a trial/not guilty plea, it’s more of a piece of string.
You absolutely don’t need a legal or managerial background, and I’d argue that either could be a disadvantage - while the presiding justice effectively manages proceedings, it generally takes four years or more to become a PJ, and as a winger, it’s much more about teamwork. And a PJ should not try to manage deliberations, other than to ensure that everyone is heard, and that procedures are followed. As for legal knowledge, you’ll have your training and various resources, but most importantly, your Legal Advisor, who will guide you, and generally set out what your options are.
Should magistrates be paid? I would say no, but nor should they be out of pocket. We are paid expenses, which can include subsistence and (iirc) loss of earnings, so in theory, any costs or losses should be avoidable.
For anyone thinking about applying, I would say go for it. Go along to a convenient court and ask to sit in the public area - courts are public buildings, so you can go to any and be allowed in; court ushers are generally very helpful, if you explain why you are there, they should direct you to the best court for seeing a variety of cases, though it’s very much luck of the draw. I would recommend observing on at least two different days, travel light, and remember that phones need to be off on or silent mode.